Texture and geochemistry of multi-stage hydrothermal scheelite in the Dongyuan porphyry-type W-Mo deposit, South China: Implications for the ore-forming process and fluid metasomatism

Abstract Scheelite in the Dongyuan porphyry-type W-Mo deposit, South China, occurs both in porphyritic biotite granite and lamprophyre. Three types of scheelite (SchA, SchB, and SchC) were identified in the porphyritic biotite granite, whereas two types (Scha and Schb) were recognized in the lamprophyre. The incorporation of rare earth elements (REE) into scheelite occurs mainly via the substitution mechanism represented by 2[VIII]Ca2+ = [VIII]REE3+ + [VIII]Na+, and additionally by 3[VIII]Ca2+ = 2[VIII]REE3+ + Ca□ (where Ca□ denotes a Ca-site vacancy). The REE patterns of SchA are similar to those of the porphyritic biotite granite, suggesting that SchA inherited its REE composition directly from the porphyritic biotite granite. Complex textures in scheelite and the similar geochemical signatures of SchB1 and SchB3 imply repeated injections of ore-forming fluids into the porphyritic biotite granite during SchB formation. The evolution of ore-forming fluids, coupled with the precipitation of apatite and early scheelite (SchA and SchB), led to SchC with more intense enrichment in middle rare earth elements (MREE). Scha and SchB share identical REE patterns, indicating precipitation from the same ore-forming fluids. Prior precipitation of zoisite caused a sharp decline in MREE and heavy rare earth element (HREE) concentrations in SchB relative to SchA. The europium anomaly in scheelite reflects an initial rise in oxygen fugacity, followed by a slight decrease from SchA to SchB and SchC. Notably, the most oxidizing conditions are recorded in Scha from the lamprophyre. A binary plot of V vs. As in scheelite from the porphyritic biotite granite mirrors the oxygen fugacity trends inferred from the europium anomaly, suggesting that the V vs. As relationship may serve as a potential fingerprint for oxygen fugacity in scheelite. The decline in the Y/Ho ratio from early to late scheelite stages likely results from self-precipitation of earlier scheelite, considering that REE-Eu (mol) and Y (mol) are incorporated into scheelite at a 2:1 ratio. Calcium necessary for scheelite formation is sourced from sericitization of calcic plagioclase in the porphyritic biotite granite and biotitization of mafic minerals (e.g., hornblende) in the lamprophyre. This study demonstrates that different host rocks significantly influence the evolutionary pathways and physicochemical properties of ore-forming fluids. Water-rock interactions are crucial at multiple stages of scheelite precipitation in the Dongyuan deposit.